An electromedical implant of the type mentioned above can be designed and/or used only for monitoring (in which case it is typically referred to as a monitoring implant), or for both monitoring and therapy (in which case it is typically referred to as a therapy implant). Contrary to therapy implants, such as implantable cardiac stimulators or the like, an monitoring implant for monitoring blood flows of a living being (patient) provides no therapy options. A monitoring implant can be useful for patients having no identified need for therapy in order to monitor the patient's blood flows. For example, the conditions for electrotherapy may not (yet) be diagnosed, or the proper type of electrotherapy to be administered may be unknown, until the patient has been monitored for an extended period using a monitoring implant. Monitoring cardiac blood flow and the epithoracic, peripheral blood flow of a living being have proven to be advantageous for determining a pulse transit time, i.e., the time difference between a time at which blood is ejected from the heart of a patient and the time of a blood pulse in the thoracic tissue, wherein the blood pulse in the thoracic tissue is caused by the ejected blood and is delayed from the time of ejection.
A variety of approaches are known for determining the pulse transit time. For example, U.S. Pat. No. 7,029,447 discloses a non-invasive system for measuring the blood pressure of a patient, wherein the pulse transit time is determined from the difference between the times of the heart ejection event and the arrival of the pulse wave in the periphery of the body using an external measuring device. The time of the cardiac contraction is determined from the externally measured thoracic impedance and the QRS complex of a surface ECG. The arrival of the pulse wave at a peripheral organ, specifically the lower arm, is determined by use of optical plethysmography. U.S. Pat. Nos. 6,748,262 and 5,743,856 describe further non-invasive methods for determining the pulse transit time based on an ECG signal and a thoracic impedance signal.
While such non-invasive systems have proven useful for monitoring purposes, they are disadvantageous and at times cumbersome. However, cardiac invasive systems are generally not a suitable option for patients having no therapy indication, i.e., no (presently) defined need for therapy. The implantation of probes in the heart, for example for determining an ECG signal using a cardiac stimulator, is complex and associated with risks for the patient. In this respect, obtaining thoracic impedance measurements via the probes of a cardiac stimulator, which can be implanted in the heart to obtain an impedance cardiogram (ICG) or electrocardiogram (ECG), is only appropriate for patients suffering from cardiac insufficiency. However, this is unsuitable for patients without an electrotherapy indication. At the same time, this group of patients constitutes a significant portion of patients suffering from cardiac insufficiency.
It would be useful to have an electromedical implant for monitoring a cardiac blood flow and an epithoracic, peripheral blood flow of a living being (patient), which can also be used to monitor a pulse transit time in the long term, without requiring cardiac invasion. In this respect, US 2009/0062667 A1 determines a pulse transit time from the peak pulse arrival times of two signals for monitoring arterial blood pressure. Using subcutaneously implanted electrodes, a first signal in the form of an electrocardiogram is determined, which is indicative of an electrical activity of a patient's heart (i.e., the blood ejected from the heart). In addition, a photoplethysmography sensor implanted in the pectoral region is used to obtain a corresponding signal which is indicative of a mechanical activity of a patient's heart (i.e., the arrival of a blood pulse in the pectoral region). This concept can be implemented as an implantable monitor having no electrotherapy device. The electrocardiogram and the photoplethysmography signal are used within an evaluation unit in order to determine an arterial blood pressure.
While measuring electric activity and mechanical activity of a patient's heart is, in principle, helpful for determining a pulse transit time in a monitoring implant, this approach is in need of improvement.